This paper presents the first detailed geologic map of in situ lower ocean crust; the product of six surveys of Atlantis Bank on the SW Indian Ridge. This combined with major and trace element compositions of primary magmatic phases in 99 seafloor gabbros shows there are both significant vertical and ridge-parallel variations in crustal composition and thickness, but a continuity of the basic stratigraphy parallel to spreading. This stratigraphy is not that of magmatic sedimentation in a large crustal magma chamber. Instead, it is the product of dynamic accretion where the lower crust formed by episodic intrusion, large-scale upward migration of interstitial melt due to crystal mush compaction, and continuous tectonic extension accompanied by hyper- and sub-solidus, crystal-plastic deformation.Five crossings of the gabbro-peridotite contact along the transform wall show that massive mantle peridotite is intruded by cumulate residues of moderately to highly evolved magmas, few of which could be even close to equilibrium with a primary mantle magma. This contact then does not represent the crust-mantle boundary as envisaged in the ophiolite analog for ocean crust. The residues of the magmas parental to the shallow crust must also lie beneath the center of the complex. This, and the nearly complete absence of dunites in peridotites from the transform wall, shows that melt transport through the shallow lithosphere was largely restricted to the central region of the paleo-ridge segment.There is almost no evidence for a melt lens or high-level storage of primitive melt in the upper 1500 m of Atlantis Bank. Thus, the composition of associated mid-ocean ridge basalt appears largely controlled by fractional crystallization of primitive cumulates at depth, near or at the base of the crust, modified somewhat by melt-rock reaction during transport through the overlying cumulate pile to the seafloor.Inliers of the dike-gabbro transition show that the uppermost gabbros crystallized at depth and were then emplaced upward, as they cooled, into the zone of diking. ODP and IODP drilling along the center of the gabbro massif also found few primitive gabbros that could have been in equilibrium with the original overlying lavas. Evidence of large-scale upward, permeable transport of interstitial melt through the gabbros is ubiquitous. Thus, post-cumulus processes, including extensive reaction, dissolution, and re-precipitation within the cumulate pile have obscured nearly all evidence of earlier primitive origins. We suggest that many of the gabbros may have started as primitive cumulates but were hybridized and transformed by later, migrating melts to evolved compositions, even as they ascended to higher levels, while new primitive cumulates were emplaced near the base of the crust. Mass balance for a likely parental melt intruded from the mantle to form the crust, however, requires that such primitive cumulates must exist at depth beneath Atlantis Bank at the center of the magmatic complex.The Atlantis Bank Gabbro Massif accreted by direct magma intrusion into the lower crust, followed by upward diapiric flow, first as a crystal mush, then by solid-state, crystal-plastic deformation, and finally by detachment faulting to the sea floor. The strongly asymmetric spreading to the south, parallel to the transform, was due to fault capture, with the bounding faults on the northern rift valley wall cut off by the detachment fault, which extended across the zone of intrusion causing rapid migration of the plate boundary to the north.

中文翻译：

---

亚特兰蒂斯银行加布布鲁地块，西南印第安岭

本文介绍了第一个详细的原位低层洋壳地质图；是西南印度洋地区亚特兰蒂斯银行（Atlantis Bank）六次调查的结果。这与99个海底辉长岩主要岩浆相的主要和微量元素组成相结合，显示出地壳组成和厚度均存在明显的垂直和平行脊变化，但基本地层的连续性与扩展平行。该地层不是大型地壳岩浆室内的岩浆沉积。相反，它是动态积聚的产物，在这种积聚中，由地壳侵入引起的下地壳，由于晶体糊状压实而引起的间隙熔体的大规模向上迁移，以及伴随超固相和亚固相线的连续构造扩展，晶体塑性变形。沿转换壁的辉长岩-橄榄岩接触的五个交叉点表明，大量的地幔橄榄岩被中等至高度演化的岩浆的累积残留物侵入，其中很少有一个岩浆甚至可以接近原始地幔岩浆的平衡。因此，这种接触并不代表洋壳类似物的蛇绿岩类似物所设想的壳幔边界。浅地壳外的岩浆残留物也必须位于复合体中心之下。这以及转换壁中橄榄岩中几乎完全没有杜尼特斯表明，通过浅岩石圈的熔体运移在很大程度上局限于古脊段的中央区域。几乎没有证据表明有熔融透镜或高晶状体。在亚特兰蒂斯银行（Atlantis Bank）的上部1500 m中对原始熔体进行水平存储。因此，伴生的中洋洋脊玄武岩的组成似乎主要受地壳深处，地壳附近或底部的原始颗粒的分步结晶控制，在通过上覆的累积堆运至海底的过程中，熔融岩的反应对其进行了一定程度的改变。堤坝-gabbro过渡表明，最上层的辉长岩在深处结晶，然后在冷却时向上放置到堤防区。沿辉长岩断层中心进行的ODP和IODP钻探也发现很少有原始辉长岩可以与原始覆岩熔岩保持平衡。大量向上，渗透性的间隙熔体通过辉长岩的证据是普遍存在的。因此，后累积过程包括广泛的反应，溶解，堆积物中的降水和再沉淀几乎掩盖了所有早期原始起源的证据。我们建议，许多辉长岩可能始于原始的累积点，但后来被杂交和转化，将熔体迁移到进化的成分中，即使它们上升到更高的水平，而新的原始累积点也被放置在地壳的底部附近。从地幔侵入形成地壳的可能是母体熔体的质量平衡，但是要求这些原始堆积物必须存在于岩浆复合体中心的亚特兰蒂斯河岸以下的深处。下部地壳，然后是向上的二进水，首先是晶体糊状物，然后是固态，晶体-塑性变形，最后是到海床的断层断裂。